Open Question: Friend isn't responding to phone calls, was supposed to return from camping today?

I had a friend go out camping in the Texas hill country with a couple of other friends over the weekend.
One of them is a fairly competent camper, and I believe the other two are at least able to hold their own. They are all relatively healthy college students and to my knowledge they took a pretty healthy water supply. They went out to Enchanted Rock and were supposed to be back today. It's now 9pm and I'm getting slightly concerned. The two whose cell phone numbers I have are out of power or service. I know that there are a vast number of reasons they could be running late. Anything from car trouble, to just hacking out an extra hike, but I don't want to wake up and find out a phone call to the authorities would have helped. When should I be worried?

Source:
http://answers.yahoo.com/question/index?qid=20110821190313AAYrWmA

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